Dual frequency antenna receiving systems are of particular interest for global positioning satellite (GPS) systems, which use signals in two different frequency bands, L1 and L2, with wavelengths corresponding to 19 cm and 24.4. cm, respectively. However, a problem with such systems is that the satellite signal reflects off of the earth's surface, producing a reflected signal that can interfere with the primary signal directly received by the antenna. Choke structures are one technique that can be used to increase the ability of the antenna to reject multipath signals.
Several constructions of choke-ring ground planes for multipath rejection in dual frequency systems are known to the art. Common single wavelength choke structures use a series of grooves with a depth equal to slightly more than one-quarter of the wavelength of the signal. If the antenna is to process signals in both L1 and L2 bands, the longer wavelength (L2) is used to set the depth of the grooves. Part of the reflected signal couples into the groove. The round trip path length through each groove is one-half of a wavelength, corresponding to a 180 degree phase inversion. These phase inverted signals of each of the grooves combine together and cancel part of the reflected wave, leading to improved antenna performance of the primary signal.
But in known designs, it is not possible to select a groove depth which provides the best multipath rejection for both of the GPS L1 and L2 frequencies. That is why usually the groove depth is chosen to provide the best rejection for the L2 band, at the expense of the L1 band. The best groove depth for L1 is approximately 30% less than the best depth for L2. Conventional thinking of the prior art teaches that decreasing the groove depth will cause deterioration of multipath rejection for the L2 band because of the appearance of a surface wave above the ground plane when the depth becomes much less than the quarter of the wavelength of L2. Therefore, in all existing choke-ring ground planes, the multipath rejection is much better for the L2 signals than it is for the L1 signals. This is why all the known choke-ring ground planes can be referred to as "single-frequency" ground planes.
T. Hekmat, et al., "Integrated GPS/GLONASS Antenna for High Performance Applications", ION GPS-95 Meeting, Sep. 12-15, 1995, discloses a choke-ring ground plane with two systems of grooves. The groove depth of one system is a little bigger then a quarter of wavelength of the L2 signal, and the groove depth of the second system is a little bigger then a quarter of wavelength of the L1 signal. But no performance characteristics were provided by the reference, so the effectiveness of this construction is not known.
As described in the Hekmet reference, other prior art constructions are based on a horn with corrugated walls. As it follows from the theory of the corrugated walls, the corrugation groove depth has to be bigger than a quarter of wavelength of the L2 signal in order to avoid the appearance of the surface wave above the ground plane, and therefore to avoid the deterioration of multipath rejection. However, this does not improve the multipath rejection for the L1 signals.
Accordingly, there is a need in the art to provide antenna systems which can receive signals in two (or more) bands, such as L1 and L2, while providing comparable multipath rejection for each of the bands. There is also need to improve the multipath rejection characteristics of antenna systems in each of the bands.